Magnesium and its alloys have been considered for answering the growing demands for using light metals in traortation industry, because It shows unique properties such as low density, high strength to weight ratio, good electromagnetic insulation, tremendous dimensional durability, high dampening and good machining. Hence, these alloys are widely used in industries such as automotive, spacecraft, and communications. However, high activity of magnesium has reduced the corrosion and wear properties of magnesium and its alloys. Plasma electrolytic oxidation is a modern surface engineering technique that has recently attracted industry’s attention for modifying the surface of light metals such as aluminum, titanium and magnesium by creating a ceramic oxide coating. Many coating parameters affect the morphology, structure and corrosion properties of these coatings. Among these, the electrolyte composition and electrical parameters such as voltage and the waveform have a significant effect on the structure and properties of these coatings. In this study, plasma electrolytic oxidation method was used to treat AZ91 magnesium alloy in an aluminate electrolyte to increase corrosion resistance. In order to evaluate the role of modification of coating composition, sodium fluoride salt was added to form a stable phase of magnesium fluoride, and also two voltages of 350 and 400 volts and three waveforms including unipolar, bipolar with 20% and 40% cathodic cycles were selected. The bipolar waveform with 40% cathodic duty cycle could receive a higher current density that indicating the ability of this waveform to continue the coating growth while the current density reached to zero in unipolar waveform and coating was stopped after 100 seconds. The presence of magnesium fluoride in all coatings was confirmed whereas the bipolar waveforms with 40% cathodic duty cycle along with 20% cathodic duty cycle could produce high content of magnesium fluoride in the coating. also, these waveforms were able to create a coating with less defects and low porosity which increased the compaction and thickness of the coating. The results of corrosion tests involving potentiodynamic polarization and electrochemical impedance spectroscopy were in agreement with the changes observed in morphology and structure, where the samples coated at 400V using the bipolar waveforms did not show an induction loop up to 3 days and were able to exhibit high corrosion resistance. Finally, the coating that formed by the bipolar waveforms with 40% cathodic cycle at higher voltages (400 volts), represent more magnesium fluoride phases and high thickness due to its modified morphology, and could reveal the highest corrosion resistance up to 28 days immersion in a 3.5 wt% sodium chloride solution, where there was no sign of underlying corrosion in this coating. Therefore, increasing the cathodic duty cycle of the bipolar waveforms and increasing the coating voltage can improve the corrosion performance of the plasma electrolytic oxidation coatings of AZ91 magnesium alloy in aluminate bath